A drilling riser is made up of an assembly of tubular elements whose length generally ranges between 15 and 25 m, assembled by connectors. The weight of the riser borne by an offshore platform can be very great, which requires suspension means of very high capacity at the surface and suitable dimensions for the main tube and the connection fittings.
So far, the auxiliary lines: kill lines, choke lines, booster lines and hydraulic lines are arranged around the main tube and they comprise insertable fittings fastened to the riser element connectors in such a way that these high-pressure lines can allow a longitudinal relative displacement between two successive line elements, without any disconnection possibility however. Owing to these elements mounted sliding into one another, the lines intended to allow high-pressure circulation of an effluent coming from the well or from the surface cannot take part in the longitudinal mechanical strength of the structure consisting of the entire riser.
Now, in the perspective of drilling at water depths that can reach 3500 m or more, the dead weight of the auxiliary lines becomes very penalizing. This phenomenon is increased by the fact that, for the same maximum working pressure, the length of these lines requires a larger inside diameter considering the necessity to limit pressure drops.
Document FR-2,891,579 aims to involve the auxiliary lines, kill lines, choke lines, booster lines or hydraulic lines, in the longitudinal mechanical strength of the riser. According to this document, the tubes that make up an auxiliary line are assembled end to end by rigid connections allowing longitudinal stresses to be transmitted between two tubes. Thus, the auxiliary line makes up a rigid assembly that affords the advantage of transmitting stresses between the top and the bottom of the riser.
One drawback of the riser according to document FR-2,891,579 lies in the overall dimensions of the connectors. FIG. 1 describes an assembly of riser sections as described by document FR-2,891,579. The connection system for assembling two riser sections consists of a connector A intended to assemble two tubes of the main line, and connecting means B1 and B2 for assembling auxiliary line elements. When connector A and connecting means B1 and B2 are arranged in the same plane, the radial dimensions E of the connection system are substantially equal to the sum of the radial dimensions EA of connector 1 and of the radial dimensions EB1 and EB2 of connecting means B1 and B2. This layout can lead to obtain large overall dimensions E that can be greater than the maximum opening diameter of the rotary table used upon assembly and when lowering or raising a riser at sea.
The present invention provides axial and radial offset of the auxiliary line connectors with respect to the main tube connector in order to reduce the overall dimensions of the riser.